Among conventional electronic sphygmomanometers, there is a sphygmomanometer that measures blood pressure according to the oscillometric method. In the oscillometric method, an arm band (cuff) is wound around a measurement site of a measured person in advance. At the measurement time, a pressure inside the cuff (cuff pressure) is increased to become higher than a systolic blood pressure, and then, is gradually decreased. In this process of decreasing the pressure, pulsation occurring in an artery in the measurement site is detected as a pulse wave signal by a pressure sensor through the cuff. The cuff pressure and a magnitude of the detected pulsation (amplitude of the pulse wave signal) at that time are utilized to decide a systolic blood pressure and a diastolic blood pressure.
As biological information relating to blood pressure values measured in this manner, there are a degree of arteriosclerosis and vessel compliance. As techniques of finding these from a blood pressure waveform, there are a method of examining a velocity at which a pulse wave ejected from a heart is propagated (PWV: Pulse Wave Velocity) (Patent Document 1: Japanese Unexamined Patent Publication No. 2000-316821 (Japanese Patent No. 3140007), Patent Document 2: Japanese Unexamined Patent Publication No. H9-122091 (Japanese Patent No. 3599858)), and a method of examining Augmentation Index (AI), which is information of a reflected wave included in a pulse wave (Patent Document 3: Japanese Unexamined Patent Publication No. 2004-195204).
The PWV is measured in a state where sensors (cuffs or the like) that measure a pulse wave or the like at two or more positions such as in upper arms, lower legs and the like are attached as in the measurement time of an electrocardiogram and a phonocardiogram. At the measurement time, the measurement is simultaneously performed using the respective sensors. The PWV is calculated from a time difference exhibited by pulse signals detected by the respective sensors, and a length of an artery between the two points where the sensors are attached.
In this manner, since for the measurement of the PWV, the sensors such as cuffs need to be attached at two or more positions, it is difficult to measure a degree of arteriosclerosis easily and conveniently.
Moreover, the AI is calculated based on a pressure pulse wave signal measured while pressing a radial artery of a wrist with an appropriate pressure. However, a mechanism to press with the appropriate pressure, and an expensive sensor unit to precisely position a measurement site are needed, and a technique for properly attaching the sensor unit is needed. These make it difficult to measure the degree of arteriosclerosis easily and conveniently.
On the other hand, as an apparatus capable of continuously measuring a blood pressure waveform noninvasively, and being used easily and conveniently, there is a sphygmomanometer of the volume compensation method type (Patent Document 4: Japanese Examined Patent Publication No. S59-005296).
The volume compensation method is as follows. That is, it is a method in which an artery is compressed by a cuff from the outside of a biological body to keep constant a volume of the artery pulsating in synchronization with a heart rate, thereby balancing a pressure (cuff pressure) that compresses a measurement site and an internal pressure of the artery of the measurement site, that is, a blood pressure, and the cuff pressure when this balanced state is maintained is detected to thereby obtain blood pressure values continuously.
Accordingly, in the volume compensation method, two points of the detection of a volume value are important—when the artery is in an unloaded state (control target value) and the maintaining of this unloaded state (servo control). As a method of the servo control, PID (referring to control in which Proportional Control, Integral Control and Derivative Control are combined to cause the volume value to converge to the control target value) of feedback control is used.
Here, in order to perform the measurement with a high accuracy, a servo gain needs to be adjusted in accordance with a control subject. In the conventional servo control, a technique of deciding the servo gain from responsiveness to input of the control subject is common. Specifically, a method is employed in which a time required until an output value starts to respond when an input value is varied in a staircase pattern (waste time), and a velocity of change from the start of response (time constant) are measured in advance, and based on these values, the servo gain is set.
However, since this method requires adjustment through trial and error, it takes time to adjust, which makes it difficult to apply this method to the blood pressure measurement in which the control needs to quickly start.
Moreover, this method is based on the premise that the responsiveness of the control subject is unchanged, which also makes it difficult to apply the method to the control of the blood pressure measurement for a biological body whose responsiveness frequently changes in accordance with change in physical condition and the like.
Consequently, in an electronic sphygmomanometer by the volume compensation method, the control is started without performing any preadjustment, and during the control, there arises a necessity to decide an optimal servo gain. In order to decide the optimal servo gain during the control, in Patent Document 4, the servo gain is gradually increased, and utilizing the servo gain when an elimination rate of an arterial volume change signal (an amplitude during control/an amplitude before control) becomes smaller than a predetermined value, the blood pressure measurement is performed. Such control is also shown in FIG. 1 of Non-Patent Document 1 (Document 1: Yamakoshi K, Shimazu H, Togawa T, Indirect measurement of instantaneous arterial blood pressure in the rat, Am J Physiol 237, H632-H637, 1979)    Patent Document 1: Japanese Unexamined Patent Publication No. 2000-316821    Patent Document 2: Japanese Unexamined Patent Publication No. H9-122091    Patent Document 3: Japanese Unexamined Patent Publication No. 2004-195204    Patent Document 4: Japanese Examined Patent Publication No. S59-005296    Non-Patent Document 1: Yamakoshi K, Shimazu H, Togawa T, Indirect measurement of instantaneous arterial blood pressure in the rat, Am J Physiol 237, H632-H637, 1979